1. Filed of the Invention
This invention relates to a drafting unit for a ring spinning device comprising two delivery rollers defining between them an exit nip for at least one drawframe-treated roving, one of said rollers forming a deflection line with a slot-shaped suction zone for the roving exiting the exit nip, which extends between the exit nip and a downstream pressure roller arranged in the sense of deflection, and comprising an air nozzle offset laterally with respect to the suction zone for a blast of air directed towards the delivery roller with a flow component transverse to the suction zone, which extends in an arc-shaped path with the air nozzle on the inside of the arc between its feed and its discharge end.
2. Description of the Prior Art
To feed a ring spinning device with a drawframe-treated, narrow roving, which has already been adapted to a large extent to the future cross-section of the yarn, it is known (AT-PS 391 328) to provide one of the two delivery rollers of the drafting unit with a slot-shaped suction length downstream of the exit nip and to arrange, at least on one side of this suction length, an air nozzle whose blast of air is directed crosswise to the suction length in a way that the drawframe-treated roving, which is retained from being displaced sidewards in the exit nip of the drafting rollers and in a guide nip of a downstream pressure, will be bunched up to form a comparatively narrow roving by the cooperation of the suction air and air blow streams. The narrow roving exiting the guide nip defined by the pressure roller facilitates the twisting of the fibres by means of the revolving urchin of the ring spinning device, because the fibres, which otherwise would be arranged laterally in a wider roving, need not be collected in a well-defined triangular area to form a circular-section roving. The smaller this spinning triangle can be kept the more favourable are the resultant conditions of twist, all the more so as the strength of the yarn adjacent to the spinning triangle is still very low because the yarn has not been twisted yet.
To improve these spinning conditions it has in addition been suggested (U.S. Pat. No. 4,953,349) to bulge the slot-shaped suction zone sidewards away from the air nozzle between its feed end and its discharge end. The protuberance of the slot-shaped suction area away from the air nozzle enables a larger tensile stress to be applied to the fibres of the roving adjacent to the outside arc than is applied adjacent to the inside arc of the suction zone which, in an interaction with the air nozzle blast of air directed against the inside of the arc, involves a marked improvement in the bunch-up of roving fibres such that the bunched roving will already be transformed, to a large extent, into a cross-sectional shape that only requires a small spinning triangle for conversion to the cross-sectional shape of the resultant yarn. Although such steps have proved to be excellent for short-staple fibres malfunctions might arise when long-staple fibres are used if, despite an increase in the diameter of the delivery roller defining the deflection length for the roving, the spacing between the exit nip of the delivery rollers and the guide slot between the pressure roller and the associated delivery roller along the suction zone is smaller than the length of single staple fibres, which then cannot join closely to the path of the roving in the suction zone area. The bunching effect will be impaired by the path of the fibres which comes about by the grip acting on either side of the fibres.